It is well known in the prior art to provide compound master brake cylinder assemblies for hydraulic braking systems. These master cylinders have a large diameter piston which initially displaces a high volume of fluid under relatively low pressure, and an associated piston of smaller diameter capable of displacing a relatively low volume of fluid at high pressure. The purpose of such systems is to insure that the brake shoes (or pads) quickly engage the associated drum (or disc), and after engagement operate through the high pressure low diameter cylinder to apply the braking force. The reason that such systems are desirable is that when an operator applies the braking force it is necessary for the cylinder within the system to operate within the operator's physical limitations. Thus, if the operator were operating solely through a large diameter piston when the brake shoes start to engage it would be necessary for him to apply a much greater force in order to bring the vehicle associated with the system to a stop. This becomes more of a problem as the weight of the vehicle increases. On the other hand, if the operator were operating solely through a small diameter high pressure piston arrangement the brake pedal travel would be unnecessarily great.
One of the problems associated with the compound master brake cylinder is maintaining relatively uniform brake pedal force, particularly during the transitional phase between the operation of the large diameter low pressure piston and the small diameter high pressure piston. Thus, it is desirable that no input pressure peaks or valleys be encountered during the operation of the brake pedal. On the other hand, it is also desirable that the pressure within the chamber behind the large diameter piston be reduced to the lowest possible level as quickly as possible in order to insure good operator efficiency.
In the past, various proposals have been made which attempt to solve these problems. One example is U.S. Pat. No. 3,228,195 issued Jan. 11, 1966. While the design shown in this patent will apparently reduce the pressure in the chamber behind the large diameter piston to provide good operator efficiency when it is necessary for only the small diameter high pressure piston to operate, the pedal effort does not remain relatively uniform, particularly during that portion of the operation of the master cylinder when fluid pressure in the chamber behind the large diameter piston is being reduced. Another example is U.S. Pat. No. 2,508,403 issued May 23, 1950. While this design will maintain more uniform pedal effort, it will not reduce the pressure behind the large diameter piston as quickly as possible while maintaining uniform pedal effort. In addition, this design is only usable with compound master brake cylinders of the type having a fixed high pressure cylinder which receives a movable piston.